Rongalite, also called Rongalit (registered trademark of BASF

BASF

BASF SE is the largest chemical company in the world and is headquartered in Germany. BASF originally stood for Badische Anilin- und Soda-Fabrik . Today, the four letters are a registered trademark and the company is listed on the Frankfurt Stock Exchange, London Stock Exchange, and Zurich Stock...

) is sodium hydroxymethylsulfinate, or Na⁺HOCH₂SO₂^-. The salt has many names, including also sodium formaldehyde sulfoxylate and Bruggolite. It is water-soluble and generally sold as the dihydrate.
This salt is prepared from sodium dithionite

Sodium dithionite

Sodium dithionite is a white crystalline powder with a weak sulfurous odor. It is a sodium salt of dithionous acid. Although it is stable under most conditions, it will decompose in hot water and in acid solutions...

:

Na₂S₂O₄ + 2 CH₂O + H₂O → NaHOCH₂SO₃ + NaHOCH₂SO₂

This reaction proceeds quantitatively, such that dithionite can be determined by its conversion to rongalite, which is far less O₂-sensitive and thus easier to handle.

The hydroxymethanesulfinate ion is unstable in solution towards decomposition to formaldehyde and sulfoxylate ion, however addition of at least one equivalent of formaldehyde pushes the equilibrium towards the side of the adduct and reacts further to give the bis-(hydroxymethyl)sulfone.